Bug: Cant use nebular/auth without providing nebular/theme

### Issue type

**I'm submitting a ...** (check one with "x")

* [+ ] bug report
* [ ] feature request

### Issue description

**Current behavior:**

Importing only nebular/auth causes an issue
NullInjectorError: R3InjectorError(AppBrowserModule)[EffectsFeatureModule -> InjectionToken ngrx/effects: Feature Effects -> [object Object] -> AuthEffects -> AuthDataAccessService -> InjectionToken Nebular Auth Strategies -> NbOAuth2AuthStrategy -> InjectionToken Window -> InjectionToken Window -> InjectionToken Window]:
NullInjectorError: No provider for InjectionToken Window!

**Expected behavior:**

serving a project without nebular/theme without issue

**Steps to reproduce:**

1) create an angular 10 project
2) import NbAuthModule.forRoot()
3) start a project

**Related code:**

```
insert short code snippets here
```

### Other information:

**npm, node, OS, Browser**
```

```
node version v14.3.0
npm 6.14.5
"@angular/core": "^10.0.4",
"@nebular/auth": "^6.0.0",
**Angular, Nebular**
```

```